Piezoelectric tranducer systems

ABSTRACT

A transducer assembly has a generally planar resonator ( 12 ) (such as a disc) bonded on one face to a sheet of piezoelectric material ( 12 B) and having a mounting flange ( 12 C) (which may extend all around the periphery of the resonator) to mount the resonator between inner ( 13 ) and outer ( 11 ) mounting elements such as rings. An electric circuit ( 24 ) drives the piezoelectric sheet ( 12 B) and, when used for sound generation, a tuned acoustic structure is provided for directing sound away from the resonator ( 12 ).

FIELD OF THE INVENTION

The present invention relates to piezoelectric transducer systems andapplications for such systems, for example operating in the audiblesound spectrum or even at ultrasound frequencies.

BACKGROUND OF THE INVENTION

Piezoelectric transducers are in common usage in numerous products. Theycontain a diaphragm typically fabricated by attaching a smaller diameterthin piezoceramic disk onto a larger diameter thin metal disk. Applyinga voltage across the piezoceramic disk produces stresses that cause thediaphragm to flex like a drum skin. By energising such a system at anaudible sound frequency, sound can be generated. Diaphragms may be usedon their own, as in wrist watches, but they are usually mounted onto anacoustic chamber with an opening to improve the acoustic power output ofthe transducer. Acoustic power output and/or sound directionality canfurther be enhanced with a suitably shaped air cavity that behaves likea horn.

One common method of mounting a diaphragm is to glue its nodal circle toa matching cylindrical protrusion that forms part of the acousticchamber. The nodal circle's length does not change as the diaphragmflexes. Nodal mounting is inexpensive and has low mounting loads butexposes only the diaphragm surface within the nodal diameter to theacoustic chamber. Another common method is to mount the diaphragm bygluing or clamping it around a small peripheral annular region. This hashigher mounting loads but exposes a larger portion of the diaphragm tothe acoustic chamber and equivalent diaphragm flexing results in thecentre deflecting further compared with nodal mounting.

One limiting factor of the acoustic power output is the tensile strengthof the piezoceramic. Ceramics are typically much stronger in compressionthan in tension. Increasing the flexing vibration of the diaphragm byapplying higher voltage waveforms increases acoustic power output. Thefatigue life of the transducer depends mainly on the maximum tensilestress experienced by the piezoceramic. Specifying a required fatiguelife for a transducer in turn determines the maximum vibratingdeflection, the maximum driving voltage waveform and the maximumacoustic power output for a given frequency.

Previously published proposals in the field of transducers include U.S.Pat. No. 6,353,277 (Han-Jose), U.S. Pat. No. 5,514,927 (Tichy) and U.S.Pat. No. 5,030,872 (Boehnke & Pieper). These specifications arerepresentative of particular configurations of transducers forparticular purposes but in recognising this prior art no admission ismade that any one item discloses any arrangement which is known or ofgeneral knowledge in Australia or any other country.

One important application envisaged for transducer arrangementsembodying the present invention is to the field of sound generators andespecially those intended to provide high volume output for alarms andapplications, such as for use by referees in sporting events. Howeveruse of the invention in other applications and fields is envisaged.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a transducer assemblycomprising

-   -   a) a resonator element having        -   i) a sheet-like main body and        -   ii) a mounting flange having a outer surface facing away            from the main body and an inner surface facing towards the            main body,    -   b) first and second mounting elements engaging respectively the        inner and outer surfaces to mount the resonator element    -   c) a piezoelectric body bonded to the main body, and    -   d) means for electrically connecting the piezoelectric body and        the resonator element to and alternating current source for        energising the resonator element and causing its resonation.

The invention also consists in assemblies further characterised by otheradditional features that can be advantageous and highly useful forparticular applications as well as apparatus embodying the transducerassembly. Such apparatus can be adapted to be used for particulardesired outputs such as an acoustic device for providing sound signalsor other applications such as displacing an operating fluid in which thetransducer assembly is in contact. In the latter case it may beappropriate to operate the transducer assembly at an ultrasonicfrequency so that applications such as fluid pumping could beimplemented a the device which could be very compact, durable, lightweight and silent to the human ear. For example, a low pressure airsupply could be provided in a device requiring air flow such as apersonal fan. In such an application the device may readily be designedwith sensors to activate the electrical circuitry to energise the systemonly when required. Thus, bulk complexity and the need for a mains powersupply might be obviated.

However, a particularly significant field of application of manyembodiments of the invention is to acoustic signals for alarm or similarpurposes, for example for use by officials in refereeing a sports event.

The present invention lends itself to embodiments having electriccircuitry to activate selected different frequency tones or patterns fordifferent purposes. For example in sporting events, a firstcharacteristic signal could be used by a referee to indicate there hasbeen a rule infringement but advantage is being played and a secondsignal could be use to stop a play.

It is most advantageous for many applications to form the main body as athin, planar, electrically conductive sheet with the mounting flangeextending peripherally around the whole of the main body.

For example, the main body can be of disc-like form with the mountingflange integral with the main body. The mounting flange can extend toform a skirt, for example, of a cylindrical form extending perpendicularto the central plane of the main body and parallel to the axis of theresonator. In such an embodiment the mounting elements can be in theform of rings which rigidly clamp the skirt, for example in aninterference fit.

In some embodiments the structure can be in the form of an electricallyconductive main body, an electrically insulating bonding layer appliedover one surface of the main body and a disc-like piezoelectric bodythereby bonded intimately to the main body. The piezoelectric bodytypically has an electrically conductive coating on its free face thatacts as a capacitive plate and facilitates the soldering of anelectrical connection wire.

By attaching the electrical connection wire close to the periphery ofthe main body the location is where minimum displacement of thepiezoelectric body occurs in use thereby minimising flexing andpotential mechanical damage to the electric wire.

In another aspect the invention extends to an acoustic emittercomprising the transducer assembly as in any one of the forms describedherein with a mounting for the transducer assembly and an electricaldrive circuit adapted to energise the piezoelectric body at an acousticfrequency, the device further having an acoustic structure defining anacoustic path for sound generated by the resonator and extending awayfrom the main body of the resonator on its side remote from thepiezoelectric body. The acoustic structure can include a rigid bodyspaced from an adjacent resonator main body and having a centralaperture which is small in area compared to the main body of theresonator.

An acoustic horn device can be matched to the transducer assembly tocontrol acoustic output along the axis of the main body. The horn can bein the form of a tapering conical shape having a lesser diameter remotefrom the main body. The axial length of the horn can be similar to thedimension across the resonator main body. The horn may have an ovalcross section.

In another aspect the present invention may be defined as providing anacoustic transducer assembly comprising a generally planar diaphragmhaving piezoelectric transducer material in a central portion and amounting flange extending from a peripheral portion transversely to thegenerally planar diaphragm, and first and second mounting elementsengaging and mounting the flange on its inner and outer sidesrespectively whereby an assembly is adapted to be mounted for acousticoutput when the piezoelectric transducer is electrically driven.

Preferably the form of the diaphragm is disc-shaped with the flangebeing a depending skirt extending approximately at right angles to thegeneral plane of the diaphragm.

In a preferred embodiment the first and second mounting elements arerespective rings and the skirt is of corresponding shape to be clampedbetween the rings in an interference fit.

However, in another form one of the first and second mounting elementsis a ring-shaped structure conforming substantially to the peripheralshape of the diaphragm and the other is a structural support element towhich the diaphragm is securely attached.

In a preferred embodiment the diaphragm is a disc of brass or similarmaterial and carries bonded thereto the piezoelectric element, the edgesof this element being spaced inwardly from the inner mounting element.However, a wide range of materials may be used in place of brass.

Advantageously the transducer assembly is adapted to be mounted on asupport base by a mounting cap which provides an acoustic chamber withan axially directed aperture through which sound is adapted to pass.

It has been found that with embodiments of the invention robustness,durability and high volume of sound generation can be achieved. Inparticular high applied voltages can be used without damage to thetransducer device. It is believed that the mounting structure proposedfacilitates resonance and simple mounting.

The inventor proposes the following explanation for the advantages whichcan be achieved in performance but this explanation is given to assistan understanding of the invention but the inventor is not to be bound bythe completeness or correctness of this explanation and theory. It isbelieved that the form of the mount provides high rigidity and thatconsequently there is very little hysteresis and energy loss comparedwith commonly used mounts. Furthermore the rigid mounting is believed toresult in greater energy storage capacity for the resonant mass/springbehaviour of the diaphragm for a given maximum piezoceramic tensilestress. Furthermore a compressive preload of the piezoceramic may beprovided for as a result of a mechanical flexing preload left over fromthe drawing operation of the assembly. It is believed this preload mayhave the effect of permitting a greater stress range for a given maximumpiezoceramic tensile stress. Thus the structure is of the nature of atight drumskin or stretched membrane although the full reasons forimproved acoustic power output in this case are not fully understood.

Embodiments of the invention can utilise low cost parts and low costassembly operation to produce inexpensive but robust devices yet for agiven size of device great acoustic power can be achieved.

In another aspect the invention may be defined as consisting in a devicehaving a transducer assembly in any one of the forms described herein,wherein the device is arranged to provide an acoustic signal at about 3kHz, the mass-spring resonance in the disc-shaped resonator isinherently at about 3 kHz, the electrical drive circuit has acapacitor-inductance resonance in the circuit of about 3 kHz, thepiezoelectric body as an inherent resonance at about 3 kHz and theacoustic structure has inherent resonance at about 3 kHz and has anacoustic transformer cavity dimensioned to transform a high pressure,small displacement in the operating fluid in the structure into a lowpressure, high displacement and high volume acoustic signal.

The inventor has observed that the characteristics of fluid such as airdisplacement is strongly influenced by the structure surrounding thefree face of the main body of the resonator and therefore the structureneeds to be designed to suite the application.

For illustrative purposes only an embodiment of the invention will nowbe described with reference to the accompanying drawings, of which:

FIG. 1 is a plan view of a transducer assembly embodying the inventionbut shown schematically;

FIG. 2 is a cross sectional view taken along the line A-A in FIG. 1;

FIG. 3 is a view on an enlarged scale of portion X in FIG. 2illustrating in cross-sectional view a preferred embodiment;

FIG. 4 illustrates in exploded view the components of a practical deviceusing the concepts of FIGS. 1-3 and including an acoustic chamber andacoustic transformer cavity; and

FIG. 5 is an axial cross sectional view showing assembly of the deviceof FIG. 4.

The illustrated transducer assembly has a transducer disc 12 having,centrally, a brass mounting plate 12A and a piezoceramic disc 12B bondedwith electrically insulating adhesive material to a first surface asbest shown in FIG. 2, the mounting plate 12A having a peripheral skirt12C which rigidly mounts the transducer disc in an interference fitbetween an inner ring 13 and outer ring 11. The preferred profile of therings is shown in the enlarged view of FIG. 3.

As showed in detail in FIG. 3, the inner ring 13 has a curved nose 13Awith a part-circular profile extending from an inclined ramp 13B to anouter wall 13C which extends parallel to the axis of the ring. The outerring 11 has a similar curved nose 11A at its lower inner cornerconnecting between a transverse wall 11B and an inner wall 11C whichalso extends substantially parallel to the axis of the ring. The profileof the rings is such that the structure shown in FIG. 3 is achieved by adrawing operation. Initially the brass disc 12 is flat and circular. Theperipheral portion is engaged by the rings which engage the periphery ofthe brass disc in an interference fit. The periphery of the brass disc12 is thus drawn down between the rings to form a rigid mounting.

FIG. 2 shows schematically an electric circuit 24 for driving the piezoassembly. A first connecting wire 20 is connected by electrical solder21 to an outer peripheral portion of the piezoceramic disc 12B. A secondflexible electrical connection wire 22 is connected by electrical solder23 to the radial wall of the inner ring 13. The electrical wires 20 and22 are connected to the electrical circuit 24 which is of conventionalform and provides an electrical drive signal. The circuit 24 includes anauto transformer arrangement and a high current electronic switch, thedevice being powered by a suitable battery.

The assembly shown FIGS. 1 to 3 is adapted to be mounted in casing 14which acts to provides an acoustic chamber 15 between the front of thebrass plate 12A and a front wall 16 which has a small diameter, axiallydirected acoustic discharge port 16A, best shown in FIG. 5.

The casing 14 can be of any suitable materials such as a plasticsmaterial and in the illustrated embodiment a flange 17 extends aroundthe periphery of the casing and has a shoulder 18 defining an undercutbehind which the transducer assembly is snap fitted and adapted to berigidly mounted. The unit including the casing is adapted to be fittedinto a mounting barrel 19 which, at its free end, has a nose cone 20 andan interior oval acoustic horn 21.

The utilisation of the horn structure shown in FIG. 5 has been found tosubstantially increase the acoustic level of the signal. It is believedthat a high pressure, small displacement signal in the acoustic chamber15 is transformed by the horn to produce a low pressure but largedisplacement signal which has high acoustic intensity. This can be agreat benefit for many applications.

With the assembly as shown in FIG. 5 provision in practice is made forthe application of power from a battery to drive a transducer andcontrol switching to activate the transducer are provided. The detailsare not shown in the drawing.

As an alternative to the embodiment shown in FIGS. 4 and 5, the outerring 11 could be omitted and instead the skirt 12C could be bonded tothe inner ring 13 and the profile of the casing 14 adjusted so as tocause the skirt and the inner ring 13 together to be fitted within theflange 17 of the casing.

A further alternative is to omit the inner ring 13 and to bond the outerring 11 to the skirt 12C.

1. A transducer assembly comprising a) a resonator element having i) asheet-like main body and ii) a mounting flange having a outer surfacefacing away from the main body and an inner surface facing towards themain body, b) first and second mounting elements engaging respectivelythe inner and outer surfaces to mount the resonator element c) apiezoelectric body bonded to the main body, and d) means forelectrically connecting the piezoelectric body and the resonator elementto and alternating current source for energising the resonator elementand causing its resonation.
 2. A transducer assembly as claimed in claim1, wherein the main body is a thin, planar, electrically conductivesheet and the mounting flange extends peripherally around the whole ofthe main body.
 3. A transducer assembly as claimed in claim 2, whereinthe main body is of disc-like form, the mounting flange is integral withthe main body and extends transversely to the central plane of the mainbody to form a skirt and the mounting elements are respective ringswhich rigidly clamp the skirt and are adapted to mount the transducerassembly in a structure.
 4. A transducer assembly as claimed in claim 3,wherein the piezoelectric body is disc-like in form and extends overmost of the main body and is bonded thereto by an electricallyinsulating compound.
 5. A transducer assembly as claimed in claim 4,wherein the electrical connecting means comprises a first electricallead connected to a radially outer face of the piezoelectric body remotefrom the main body and a second electrical lead connected to at leastone of the rings which are electrically conductive.
 6. A transducerassembly as claimed in claim 5, wherein the rings engage the skirt in aninterference fit and the skirt extends substantially parallel to theaxis of the main body with a smoothly curved junction portion joiningthe main body and the skirt, with the inner ring having a correspondingcurved shoulder for supporting the junction portion.
 7. A transducerassembly as claimed in claim 1, wherein the assembly is of circular formand of dimensions of about 2 cm diameter and 2 mm axial depth.
 8. Anacoustic emitter comprising a transducer assembly as claimed in claim 1,a mounting, an electrical drive circuit adapted to energise thepiezoelectric body at an acoustic frequency and an acoustic structuredefining an acoustic path for sound generated by the resonator andextending away from the main body of the resonator on its side remotefrom the piezoelectric body.
 9. An acoustic emitter as claimed in claim8, wherein the electric drive circuit includes connectors for connectionto a battery, control circuitry and an inverter for supplying analternate current supply at about 3 kHz.
 10. An acoustic emitter asclaimed in claim 8, wherein the acoustic structure includes a rigid bodyspaced from and adjacent to the resonator main body and having a centralaperture which is small in area compared to the main body of theresonator.
 11. An acoustic emitter as claimed in claim 8, and furthercomprising a horn device acoustically matched to the transducer assemblyto control the acoustic output along the axis of the main body.
 12. Anacoustic emitter as claimed in claim 11, wherein the horn issubstantially a tapering conical shaped body having its lesser diameterremote from the main body and of a length similar to the dimensionsacross the resonator main body.
 13. An acoustic emitter as claimed inclaim 12, wherein the horn has an oval cross-sectional shape.
 14. Anacoustic emitter as claimed in claim 8, wherein the device is arrangedto provide an acoustic signal at about 3 kHz, wherein the mass-springresonance in the disc-shaped resonator is inherently at about 3 kHz, theelectrical drive circuit has a capacitor-inductance resonance in thecircuit of about 3 kHz, the piezoelectric body has an inherent resonanceat about 3 kHz and the acoustic structure has inherent resonance atabout 3 kHz and has an acoustic guide wall dimensioned to transform ahigh pressure, small displacement in the operating fluid in thestructure into a low pressure, high displacement and high volumeacoustic signal.
 15. A device for displacing a fluid comprising a) amounting structure, b) means for mounting a transducer assembly asclaimed in claim 1, c) means for admitting an operating fluid intocontact with the face of the main body of the resonator element remotefrom the piezoelectric body and means for displacing the fluid to aremote location after interaction of the resonator when energised, andd) means for energising the piezoelectric body.
 16. An acoustictransducer assembly comprising a generally planar diaphragm havingpiezoelectric transducer material in a central portion and a mountingflange extending from a peripheral portion transversely to the generallyplanar diaphragm, and first and second mounting elements engaging andmounting the flange on its inner and outer sides respectively whereby anassembly is adapted to be mounted for acoustic output when thepiezoelectric transducer is electrically driven.
 17. An assembly asdefined in claim 16 and wherein the diaphragm is disc-shaped with theflange being a depending skirt extending approximately at right anglesto the general plane of the diaphragm.
 18. An assembly as defined inclaim 17 and wherein the first and second mounting elements arerespective rings and the skirt is of corresponding shape to be clampedbetween the rings in an interference fit.
 19. A transducer assembly asclaimed in claim 6, wherein the assembly is of circular form and ofdimensions of about 2 cm diameter and 2 mm axial depth.
 20. An acousticemitter as claimed in claim 9 and wherein the acoustic structureincludes a rigid body spaced from and adjacent to the resonator mainbody and having a central aperture which is small in area compared tothe main body of the resonator and further comprising a horn deviceacoustically matched to the transducer assembly to control the acousticoutput along the axis of the main body and the horn is substantially atapering conical shaped body having its lesser diameter remote from themain body and of a length similar to the dimensions across the resonatormain body.
 21. An acoustic emitter as claimed in claim 20 and whereinthe device is arranged to provide an acoustic signal at about 3 kHz,wherein the mass-spring resonance in the disc-shaped resonator isinherently at about 3 kHz, the electrical drive circuit has acapacitor-inductance resonance in the circuit of about 3 kHz, thepiezoelectric body has an inherent resonance at about 3 kHz and theacoustic structure has inherent resonance at about 3 kHz and has anacoustic guide wall dimensioned to transform a high pressure, smalldisplacement in the operating fluid in the structure into a lowpressure, high displacement and high volume acoustic signal.